This invention relates to a flight or motion simulator display, and more particularly to continuously clipping data which is currently not within the display area.
Heretofore, clipping of polygon faces outside a pyramid of vision has been accomplished by continuously determining the distance or window coordinate between each pair of adjacent vertices within each polygon face, and each boundary plane forming the pyramid of vision. The side of each plane outside the pyramid of vision was negative (non visible) and the side including the pyramid of vision was positive (possibly visible). The sign bits of each window coordinate for each vertex were collected into a single data word--the "outcode". In the case of a square window, each outcode (O.C.) contained four sign bits. In a more recent approach, the clipping of the current face, the O.C. of each vertex was "tested" separately against each of the four boundary plane. Testing is the process of determining whether a vertex is on the plus or minus side of a particular boundary plane, and is accomplished by examining the appropriate sign bit. The O.C. contains sufficient information to determine the visibility of the corresponding vertex. If each bit of the O.C. is positive, then the vertex is within the pyramid of vision and visible. Testing is a procedure for gathering the visibility information. A four sided polygon face tested against a four sided pyramid of vision required 4.times.4+(4)=20 separate tests or examinations. The number of testing steps remained the same for faces entirely within view (Case A) partially in view (Case B) or entirely out of view (Case C). There was no reduction in the number of testing steps for the simple case A and case C.+-.situations. However, more complex faces (and viewing volumes) required a proportionately greater number of tests.
In general case, the number of testing steps Tn required to clip a face was: EQU Tn=(Vn).times.(Pn)+(closing tests)
where
Vn is the number of vertices in the face, and PA1 Pn is the number of clipping planes forming the pyramid of vision
This prior art-separate test clipping technique is shown in more detail in U.S. Pat. No. 3,639,736 and U.S. Pat. No. 3,816,726, both to Sutherland; and in an article by Sutherland and Hodgman entitled "Reentrant Polygon Clipping" appearing in "Graphics and Image Printing" (Communications of the ACM), January 1964, Volume 17 #1.
FIGS. 2-7, and 9 are identical to the corresponding Sections and Figures in U.S. patent application Ser. No. 869,210, filed 13 Jan., 1978, by Sullivan et al, entitled "Real-Time Simulation of a Polygon Face Object System as Viewed by a Moving Observer," and assigned to the present assignee.